This invention relates generally to the field of electrical power supplies and more particularly to a capacitive boost circuit for lengthening the time available for a computer to execute a power down sequence when its power source is interrupted.
A problem encountered with many electronic systems and especially computers is an interruption of the operation of their power source. Such an interruption can cause loss of information stored in volatile memories and make it difficult, if not impossible, to restore the computer to its preinterruption state. If the interruption is detected early enough, a power down sequence can be executed to store the contents of vital operational components, such as the program counter, address registers, instruction registers, stack pointers, and cache, in non-volatile memory. The contents of these operational components can then be restored upon return of adequate power.
Although computers normally operate with a nominal power source voltage level, they can properly operate at power source voltage levels ranging from a maximum operating voltage down to a minimum operating voltage. One way to detect a likely power source interruption, then, is to sense when the power source voltage level drops to a minimum normal voltage. The minimum normal voltage is selected to be somewhat greater than the minimum operating voltage. If the power source voltage level drops to the minimum normal voltage, the computer's normal operation is interrupted. The power down sequence is then executed, and, it is hoped, completed before the power source voltage level drops to the minimum operating voltage. The time available for the power down sequence, which is the time it takes for the power source voltage level to drop from the minimum normal voltage to the minimum operating voltage, is called the hold-up time.
For new, advanced computers having more operational components, it becomes necessary to increase hold-up time. One way to lengthen hold-up time is to connect an electrolytic bulk capacitor in parallel with the power source. The bulk capacitor stores electrical energy (that is, it is charged) during normal operation of the power source. When the power source is interrupted, the stored energy is retrieved (that is, the bulk capacitor discharges) to provide the necessary energy to allow the computer to execute the power down sequence. With this arrangement, the length of time available for the power down sequence thus depends upon the amount of energy which can be retrieved from the bulk capacitor.
The amount of energy which can be retrieved from a capacitor depends upon the amount of energy stored and the proportion of stored energy that is extracted. The amount of energy stored in a capacitor depends upon its capacitance rating and the voltage level at which it is maintained. Thus, to increase hold-up time, either the bulk capacitor's capacitance or the voltage at which it is maintained must be increased. To increase capacitance, either a physically larger capacitor or additional capacitors must be used. On the other hand, if the voltage at which the capacitor is maintained is increased, the capacitor must be provided with a higher peak-voltage rating, which, in turn, also requires increased physical size. Accordingly, to increase hold-up time requires physically larger bulk capacitors.
Some circuits provide lengthened hold-up time by using a plurality of bulk capacitors. These capacitors are connected in parallel with the power source during its normal operation. When the power source voltage drops, the bulk capacitors are used to provide an elevated voltage to the load.
We have found that neither of these approaches makes maximum efficient use of the bulk capacitor's storage capability. Specifically, since the bulk capacitors are connected in parallel with the power source, they must have a peak-voltage rating at least as high as the maximum voltage provided by the power source. Because they usually begin to discharge from near the nominal power source voltage, only a fraction of the energy retrieved from them is used during hold-up time. In particular, only the energy retrieved from the bulk capacitor while the power source voltage level is dropping from the minimum normal voltage to the minimum operating voltage is truly available to lengthen hold-up time. Otherwise stated, the energy which the capacitor supplies while discharging from the nominal power source voltage to the minimum normal voltage is not used to lengthen hold-up time.
Another problem occurs with such circuits during a cut back in the power available from the local utility. In such a situation, known as a power brown-out, the power source may provide a reduced voltage above the minimum normal voltage but below the nominal power source voltage. This reduced voltage is sufficient to power the computer but may be insufficient to fully charge the bulk capacitors. The amount of hold-up energy retrieved from the bulk capacitors in this situation may be insufficient to sustain the computer for the entire power down sequence.